1. Field of the Invention
The present invention relates to a sliding structure for a shaft member and an injector.
2. Description of the Related Art
A sliding structure in which a shaft member is retained slidably in a guide hole is used in various kinds of equipment, and an injector making up a fuel injection device in an internal combustion engine is an example of such equipment. An injector switches between fuel injection and termination of the injection, for example, by axially displacing a needle made up of a shaft member and inserted into a nozzle to which a fuel for injection is supplied. In this case, the injector has a sliding structure in which the needle is retained slidably in a guide hole. The needle opens a valve, for example, when the fuel pressure in the nozzle, which always acts in the direction to open the valve, exceeds the spring force of a spring which specifies a pressure to open the valve.
There is another example of a structure, such as an injector used in a common rail type fuel injection device, in which the back pressure of the needle is switched between the high-pressure side and the low-pressure side in order to activate the needle. Such an injector has a sliding structure in which a valve chamber, provided with a valve body for isolating a back pressure chamber from a low pressure source, is provided on the way in a flow path for releasing the high-pressure fuel to be introduced into the back pressure chamber which generates a back pressure, to the low pressure source and a piston for pressing the valve body is retained in a guide hole penetrating through the wall of the valve chamber. The piston is pressed and driven by an actuator made up of a piezo stack or the like, the isolation between the back pressure chamber and the low pressure source is cancelled by displacing the valve body, and the back chamber is open to a low pressure.
In the cases of these injectors, part of fuel penetrates into the gap between the side surface of the shaft member and the side surface of the guide hole and forms an oil film therein and, therefore, the slidability is improved because of the reduction in sliding friction, but a leak of fuel from the sliding portion (leak of the sliding portion) reduces the force of the needle to open the valve and the generated oil pressure and, therefore, it is necessary to sufficiently ensure the sealing ability against the fuel leak as well as maintaining the slidability and only a clearance as narrow as several μm is allowable between the side surface of the guide hole and the side surface of the needle which are in slidable contact with each other. Because of this, there is an attempt in which labyrinth grooves are formed in the side surface of a needle to prevent the lack of an oil film and capture foreign matter in a fuel (see Patent document 1, etc.)
There is another attempt, in which the unevenness of the slidability and the sealing ability is improved by increasing the intervals at which grooves are arranged so that the interval on the low-pressure side is longer than that on the high-pressure side and the interval on the central portion is longer than that on the low-pressure side, with it being taken into account that the inner diameter of a guide hole is most enlarged and deformed on the high-pressure side thereof and the clearance becomes relatively large, whereas the degree of enlargement and deformation is small on the central portion of the guide hole, because a large slope of pressure is produced in the direction of the guide hole in a device such as the above-mentioned common rail type fuel injection device, in which a high-pressure fuel is supplied to the inside of a nozzle body (see Patent document, etc.).
[Patent Document 1]                Japanese unexamined Patent Publication (Kokai) No. 7-103106        
[Patent Document 2]                Japanese unexamined Patent Publication (Kokai) No. 2001-280223        